The Development of Life on Earth

This lecture examines how life on Earth first developed, and how this
life dramatically changed the conditions of Earth's primordial
surface. Some of the material is covered in Universe, Sections 8.5
and 30.1.

Definition of Life

To discuss the development of life, it is first necessary to define
what life is. This is a difficult task, since living organisms exhibit
so many diverse properties and behaviours. However, a simple working
definition, which encompasses most organisms, is as follows:

Living organisms can react to their environments and heal themselves when damaged.

Living organisms can grow by taking nourishment from their surroundings, and processing it into energy.

Living organism can reproduce, passing along some of their characteristics.

Living organism have the capacity for genetic change, allowing them to evolve.

Competing Theories

The two main competing theories for the development of terrestrial
life are chemosynthesis and panspermia. The chemosynthesis
theory maintains that life formed by the progressive assembly, on
Earth, of more- and more-complex organic molecules and structures,
until a point was reached where these molecules and structures
amounted to living organisms.

In contrast, the panspermia theory maintains that the same assembly
took place, but elsewhere in the Universe: either in space, or
on the surface of another planet. Some mechanism (e.g., comets) was
then responsible for seeding Earth with life or its precursor
molecules.

Of the two theories, chemosynthesis is the preferred one, since
panspermia appears to have significant flaws. This lecture therefore
focuses on how chemosynthesis accounts for life on Earth. Panspermia
will be covered in the next lecture.

The Primordial Earth

After its formation from planetesimals (see [link:diploma-1|Lecture
1]), the Earth would have undergone chemical differentiation,
where the heavier elements sink to the core and the lighter elements
rise to the surface. Among these lighter elements were traces of
hydrogen and helium, which would have given rise to a thin
primordial atmosphere.

Bombardment of primordial Earth

Since Earth's gravity is too low for it to retain hydrogen and helium,
these gasses would quickly have evaporated into space, leaving a
bare rocky globe with no atmosphere or oceans. However, the
contraction of the Earth under its own gravity, plus the decay of
radioactive elements and bombardment by meteorites, would have then
led to significant volcanic activity. This activity forced out
gasses from the interior to form a dense evolutionary
atmosphere, comprised mainly of water vapour, carbon dioxide and
nitrogen.

The Formation of Chemical Building Blocks

As the Earth cooled, much of the atmospheric water would have
condensed and fallen as rain, creating large oceans. Dissolved
in the rain was carbon dioxide, which formated of carbonate rocks such
as limestone and and marble. Through this process, most of the carbon
dioxide was removed from the atmosphere, allowing the Earth to escape
from a possible runaway greenhouse effect (see [link:diploma-2|Lecture
2]).

The next step in the development of life was the formation of simple
organic molecules. In a famous experiment conducted in 1952,
Stanley Miller and Harold Urey exposed a mixture of
gaseous hydrogen, ammonia, methane and water to an electrical arc for
a week. At the end of the experiment, the reaction chamber was coated
with a reddish-brown rich in amino acids and other compounds
essential to life.

The Miller-Urey experiment

The Miller-Urey experiment demonstrated how lightning may have
converted the evolutionary atmosphere into a living atmosphere,
rich in the chemical building blocks of life. However, it is important
to understand that the experiment did not create life! A number
of further steps, which have not yet been demonstrated experimentally,
are required before life is formed.

The Formation of Macromolecules

After the formation of the amino acids and other building blocks, and
their subsequent solution in liquid water, various processes (such as
adsorption on clay particles, or confinement in evaporating pools)
would have conspired to concentrate these compounds. Under the
influence of an energy source (such as UV light or heat), the
concentrated compounds would have combined to form large
macromolecules, such as polypeptides (precursors of
proteins) and polynucleotides (precursors of DNA).

The Formation of Prebionts

Once macromolecules had formed, the next step in the development of
life would have involved their organization into bodies with definite
shapes and chemical properties. One example is coacervate
droplets, which may be the early ancestors of cells. These
coacervates consist of macromolecules surrounded by a shell of water
molecules, whose rigid orientation makes them form a primitive
membrane. This membrane is highly selective, allowing only
certain molecules to pass though; it therefore creates a sheltered
chamber in which complex chemical reactions can develop. Have a look
at http://www.indiana.edu/~ensiweb/lessons/coacerv.html for a
description of how coacervates can be created in the lab.

Microscope image of coacervates

The Formation of Prokaryotic Organisms

With ever-more complex reactions taking place in prebionts, a point
was reached where self-replicating molecules were formed. One example
is the nucleic acids, such as DNA and RNA. These molecules have
the ability to copy themselves, and therefore act as information
stores.

Due to random mutations occurring during the copying process, the
appearance of self-replicating molecules meant that the prebionts
began to evolve through the process of natural selection. Only
those prebionts which were able to make the best use of the available
sources of energy and raw materials were able to survive and produce a
new generation of prebionts, containing the genetic information
of their own "parents".

At the point, the prebionts had reached a level of advancement which
amounted to living organisms (albeit primitive). These
single-celled organisms were prokaryotic, meaning that they
lacked an inner membrane around a nucleus of genetic material. They
were much like present-day bacteria.

The Evolution of Autotrophs

The first cells were heterotrophs, meaning that they obtained
their energy and raw materials (i.e., food) from their
surroundings. Early on in their existence, the supply of these
resources would have run short, amounting to a famine. This
famine exerted extreme evolutionary pressure on the heterotrophs,
leading quite quickly to the development of cells which were able to
produce their own food via photosynthesis.

These new autotrophs (meaning that they create their own food,
rather than relying on their surroundings) would have at first relied
on a variant of photosynthesis based around hydrogen
sulphide. Unfortunately, the supply of hydrogen sulphide is rather
limited on Earth, being found only around areas of volcanic
activity. Therefore, some autotrophs (the cyanobacteria)
subsequently made the leap to using water instead, which is of
course in great abundance.

Colonies of cyanobacteria

The Evolution of Aerobic Organisms

When photosynthesis is based around water, it produces a significant
by-product: oxygen. Since oxygen was highly toxic to the cyanobacteria
producing it, they were forced to evolve means of protecting
themselves from it, primarily by excreting it as a gas. Their success
in this led to the steady pumping of oxygen into the Earth's
atmosphere.

Initially, the oxygen would have reacted with surface minerals to
create oxides. This would have gone on until about 2 billion
years ago (see Universe, fig. 8.22), when all of the available
minerals were already oxidized. At this juncture, the levels of
atmospheric oxygen would have begun to rise, and a new type of
heterotrophic life evolved to take advantage of the oxygen as an
energy source: the aerobic respirators.

The Evolution of Eukaryotic Cells

Around 1.5 billion years ago, eukaryotic organisms first
appeared. Unlike prokaryotic organisms, these possessed inner
membranes around a nucleus of DNA, and also contained
sophisticated organelles such as mitochondria (for aerobic
respiration) and chloroplasts (for
photosynthesis). Subsequently, the eukaryotic cells developed into
specialized colonies, and provided the basis for all multicellular
life known today.

A comparison of prokaryotic and eukaryotic organisms

The Present-Day Atmosphere

Until about 400 million years ago, the levels of oxygen in the
atmosphere were steadily growing. However, at this point the amount of
oxygen produced by the photosynthetic autotrophs was balanced by the
amount consumed by the aerobic heterotrophs, and the growth
stopped. Since then, the composition of the Earth's atmosphere has
remained relatively unchanged. This present-day atmosphere has a
composition of about 20% oxygen, 78% nitrogen, and small amounts of
water vapour and carbon dioxide.